A high intensity discharge (HID) lamp has become a dot light source of the third generation after the incandescent lamp and the fluorescent lamp due to a lot of advantages such as high luminous efficiency, long lifetime and wide power range of the HID lamp. The HID lamps are widely applied in indoor and outdoor illumination environments such as plazas, docks, workshops and roads. However, both end electrodes of the HID lamp conduct no electricity in a normal state, an activating pulse of a high voltage is required to ignite the HID lamp. The HID lamp needs a ballast that provides an output voltage of 200-300 volts for forming a stable electric arc in addition to an ignition pulse. After the electric arc is generated, a high pressure gas mixture formed by metal halide and mercury vapor in the lamp may emit usable lights of a spectrum similar to the solar spectrum during the temperature raising stage. Once the electric arc is generated, the ballast must limit a magnitude of a current. Otherwise, the electric arc may result in a high current, which will damage the ballast and the lamp.
The structure of the ballast for the HID lamp can be referred to FIG. 1. FIG. 1 is a block diagram showing a conventional three-stage ballast module, which includes three portions: a power factor circuit (PFC) 101, a DC-DC converter circuit 102, and an inverter circuit 103. The DC-DC converter circuit 102 is a buck structure. The inverter circuit 103 is usually a full-bridge or half-bridge circuit. In order to reduce the cost and size of the ballast, the DC-DC converter circuit and the inverter circuit can be combined together, as shown in FIG. 2, a two-stage ballast comprises a power factor circuit 201 and an inverter circuit 203. An implementation structure of the two-stage ballast can be referred to FIG. 3. In this example, the power factor circuit 201 comprises an inductor L1, a field effect transistor (FET) S1 and a diode D1. The inverter circuit 203 utilizes a half-bridge structure. An inductor L2 and a capacitor C3 form a filter for filtering off a high frequency switching signal. According to the requirement for controlling the HID lamp, a current is controlled at a constant level during the lamp electrodes are heated. After a lamp resistance attains a stable value, the lamp power is controlled to be constant by adjusting the level of the lamp current. Therefore, it is necessary to control the current level of the HID lamp. Further, the inductor current is kept to operate in a critical continuous inductor current mode by detecting a zero point signal of the inductor current in the circuit, and thereby improving the efficiency of the HID lamp.